Ultraviolet-C radiation (UVC) does not reach Earth's surface from the sun, because all UVC is blocked either by oxygen gas (02) or by ozone gas (03) in the atmosphere. All UVC from the sun is completely absorbed by the atmosphere before reaching the surface of Earth, so interaction of solar UVC with living things and with everyday articles, such as wood, is not a natural phenomenon.
Radio waves, microwaves, and infrared light; visible light, ultraviolet light, x-rays, and gamma rays are all electromagnetic radiation (ER) with differences in the wavelength of the ER imparting the vastly different properties of each of these particular types of ER within the ER spectrum. UVC radiation, accordingly, has very unique and specific properties that are uniquely useful in view of the entire ER spectrum, and uniquely useful even compared with other types of UV radiation, such as UVA and UVB. UVC has a higher energy than UVA and UVB, which do reach the surface of Earth from the sun, while UVC does not reach the Earth, as noted above, but can be artificially produced. UVC is electromagnetic radiation with a wavelength between 100-290 nanometers (nm), or in other units, ER with an energy between approximately 4.43-12.4 electron volts (eV). UVC causes damage to the nucleic acids of microorganisms, preventing their replication, thereby destroying the microorganisms as pathogens of human disease. Hence, UVC can be used as a germicidal disinfectant. With its higher energy, UVC can also uniquely drive photolytic, photoreductive, and photooxidative chemical reactions that cannot be driven by UVA and UVB.
Wood that is milled from trees can “weather” when left exposed outside to UVA from the sun. This modification of the color of wood can sometimes be desirable, but takes a long time to achieve, for example six months, when relying on sunlight to provide UVA for discoloring the wood. UVC, however, does not naturally reach the surface of the Earth to modify wood or kill microorganisms, as described above.